1. Field of the Invention
The present invention relates to a signal amplifier circuit which amplifies an electrical signal such as an audio signal or the like.
2. Description of the Related Art
Electronic devices having an audio signal reproduction function, such as CD players, audio amplifiers, and car stereos, or portable radios, portable audio players, and the like, are generally provided with a volume control for adjusting sound volume, an equalizer for adjusting frequency response, or the like. Control of the volume or equalizer is carried out by varying audio signal amplitude.
An audio signal is amplified by an amplifier, and is ultimately outputted as sound from a speaker or headphone, which are electroacoustic transducer devices. Adjustment of volume is realized by controlling amplifier gain, or controlling attenuation rate of an attenuator. For example, Patent Documents 1 and 2 disclose an electronic volume circuit in which gain of an amplifier or attenuation rate of an attenuator are realized by switching resistance value of a variable resistor. Furthermore, Patent Document 3 discloses technology for controlling an electronic volume circuit of this type.    Patent Document 1: Japanese Patent Application Laid Open No. 2005-117489    Patent Document 2: Japanese Patent Application Laid Open No. 2005-217710    Patent Document 3: Japanese Patent Application Laid Open No. 2004-222077    Patent Document 4: Japanese Patent Application Laid Open No. H11-340759
When sound volume is changed from a certain initial value to a certain target value, a resistance value of a variable resistor of an electronic volume control is switched from a value corresponding to the initial value to a value corresponding to the target value. At this time, if the resistance value is rapidly switched, voltage value of an audio signal outputted from an amplifier changes rapidly and a signal component occurs in an audible spectrum, which causes noise to be generated. This problem can occur even in cases other than those of audio signals.